The present invention relates to the production of dinitrotoluene by toluene nitration with nitrating acid under adiabatic conditions.
It is known to convert toluene to dinitrotoluene (DNT) adiabatically (EP-A 597,361). Toluene is reacted adiabatically with at least 2 equivalents of a nitrating acid satisfying specific compositional requirements. A final temperature greater than 120.degree. C. is reached. After phase separation at this temperature, the acid phase is reconcentrated (flash evaporation under vacuum). The heat of the acid phase is used for the reconcentration. The reconcentrated acid is made up with nitric acid and is recycled to the process.
An inherent difficulty in this process is the passage of a certain amount of DNT which is dissolved in the acid with the water which distills off in the flash evaporation. This residual DNT solidifies under the water condensation conditions (the isomer mixture solidifies at approximately 55.degree. C.) and fouls the heat exchanger. There are two possible solutions to this problem.
In one of the possible solutions, staggered heat exchangers may be used. These staggered heat exchangers are operated alternately. When some of the exchangers have been fouled, they are idled and the alternate exchangers are used. The DNT present in the fouled exchangers is then melted off while those exchangers are idle. Fouling of exchange surfaces results in rapid deterioration in cooling performance. Frequent changeovers are therefore necessary. Extra power is required to melt the DNT from the idle condenser (heating up and cooling).
In a second possible solution to the problem of fouling, the heat exchanger, a co-condenser or injection condenser may be used to condense solid-forming exhaust vapors (R. A. Vauck, H. A. Muller, Grundoperationen chemischer Verfahrenstechnik [Basic Chemical Process Engineering Operations], 5th edition, VEB Leipzig 1962, p. 447). In this process, the exhaust vapors are introduced into a cold water jet spray and DNT is segregated in finely divided form as a solid. Co-current and counter-current operation are possible. Because of the large volumes of water required in this procedure, the water is circulated in a loop and cooled in the return branch. In order to separate the DNT from the water, a portion of the flow is passed out through a lock. This increases the risk of blocking lines and nozzles with low-melting organic components which tend to adhere to such lines and nozzles. Recovery of DNT in pure form requires a significant amount of energy because the DNT must first be melted down before it is recovered.
A considerably simpler and more elegant solution to the problem of fouled heat exchangers is an isothermal two-stage production of dinitrotoluene such as that disclosed in Ullmann, Encyklopadie der technischen Chemie [Encyclopedia of Industrial Chemistry], 4th edition, vol. 17, p. 392, Verlag Chemie, Weinheim (1979). In this two-stage process, an isomeric mixture of mononitrotoluene (MNT) is first prepared. This isomeric mixture is converted in a second, separate process step to an isomeric mixture of dinitrotoluene. The problem of fouling of the heat exchanger when reconcentrating the spent acid under vacuum is eliminated by injecting MNT from the first stage into the exhaust vapors (DE-A 3,409,719). The MNT which is injected lowers the melting point of the DNT thereby ensuring that the exhaust vapors remain fluid even under the water condensation conditions. The organic phase which is isolated by phase separation is recycled into the reaction.
This elegant solution is not useful in a single-stage adiabatic process for toluene dinitration (EP-A 597,361) because there is no freely available MNT present in the single-stage process. MNT is a non-isolatable intermediate product in such a single-stage process.